I can think of a few things your PCs might do to weaken an elder (or maybe 'eldrich', given that your villain isn't all *that* old) evil before the final battle.
1) The Frostfell Queen might have external power sources. She probably needed a whole lot of help to steal an entire god's power, and the items and places of power that she used probably still exist. Some of them might still feed her power, and destroying or subverting them will probably weaken her. You could have your characters have to find and destroy a ring of stones that she used to trap a portion of the old god's power, for example.
2) Some of the elements of that ritual probably still have a connection to her, even if they don't still directly feed her power. Your heroes might use them to get past her defenses or to blunt and/or weaken her attacks. For example, the knife she used to sacrifice the old god's avatar (or high priest, or whatever) would probably cut right through whatever protections she has, or the vestments from that old ritual might be enchanted to protect against the power she stole so she could survive the ritual to steal it in the first place.
3) Turnabout is fair play. Your PCs, or their allies (or even "allies" of convenience) could steal some or all of her power in turn, maybe even using the items from 1) and 2) for extra fun. You know Orcus will want a piece of her; it's an open question whether the PCs can trust him long enough to weaken her. Or maybe some fragment of the old god exists and wants payback.
4) Your new evil has made foes, not all of whom are greedy rivals or victims. This would be your "assistance of Pelor" would come into play. An epic quest to recruit Pelor directly, maybe having to take on some of his burdens to free him up to help would be interesting.
5) There might also be natural phenomena and/or pre-existing items that are harmful to her because they embody principles that oppose her. Imagine luring her into a summer-aligned region of the Feywild, for example, for a double dose of "life" and "summer" to oppose her undeath and winter, for example.